EP0264408A1

EP0264408A1 - Inductive proximity sensor

Info

Publication number: EP0264408A1
Application number: EP19870902488
Authority: EP
Inventors: Michael FÖRSTER; Gerhard Weber; Hans Peter Hentzschel; Alfred Leiter
Original assignee: Wassermesserfabrik Andrae GmbH and Co
Current assignee: Wassermesserfabrik Andrae GmbH and Co
Priority date: 1986-04-09
Filing date: 1987-04-08
Publication date: 1988-04-27
Also published as: WO1987006354A1; DE3611862A1

Abstract

Le détecteur de proximité par induction possède un circuit oscillant à cristaux liquides excité par des impulsions électriques et pouvant être amorti par l'approche d'un objet électriquement conducteur, ainsi qu'un circuit d'évaluation (33, 34) permettant de déterminer l'amortissement des oscillations provoqué par l'élément d'amortissement. L'inductance du circuit oscillant à cristaux liquides est produite par l'enroulement primaire (L1) d'un transformateur dont l'enroulement secondaire (L2) présente un nombre de spires supérieur à l'enroulement primaire (L1). Le circuit d'évaluation (33, 34) est situé dans le circuit de l'enroulement secondaire (L2).The proximity proximity switch by induction has an oscillating liquid crystal circuit excited by electric pulses and which can be damped by the approach of an electrically conductive object, as well as an evaluation circuit (33, 34) for determining the damping of oscillations caused by the damping element. The inductance of the liquid crystal oscillating circuit is produced by the primary winding (L1) of a transformer whose secondary winding (L2) has a number of turns greater than the primary winding (L1). The evaluation circuit (33, 34) is located in the circuit of the secondary winding (L2).

Description

Inductive Na __. Erurigssen.sor

description

The invention is based on an inductive proximity sensor with the features specified in the preamble of claim 1. Such a proximity sensor is known from DE-OS 33 18 900. It is a proximity sensor with low power consumption, which has an electrical LC resonant circuit, the coil of which can be influenced by the proximity of an electrically conductive object. The oscillating circuit is excited to oscillate by a direct current pulse and the degree of approximation is deduced from the decay process of the damped oscillation thus generated.

This proximity sensor therefore has a relative. low power consumption, because its oscillating circuit is not permanently excited to oscillate by an oscillator, but only by short direct current pulses, which occur at periodic intervals, the time interval of which must be so large that even when the excited oscillation decays slowly (if namely, the attenuator has the least influence on the coil of the resonant circuit) the vibration has already subsided when the next DC pulse occurs. The duration of the DC pulse should be long enough to enable the capacitor of the LC resonant circuit to be sufficiently charged, but it should also be short

be compared to the period of the resonant circuit frequency, so that no disturbing current begins to flow through the coil of the resonant circuit when charging the capacitor of the resonant circuit.

The oscillating circuit begins to oscillate from the energy stored in the capacitor of the oscillating circuit by means of the direct current pulse. The vibration is damped. The decay time depends on the losses of the resonant circuit, in particular on the losses of the coil of the resonant circuit. If the coil is damped from the outside by the attenuator, the decay time is shortened.

According to DE-OS 33 18 900, the decay process of the damped oscillation can be evaluated in various ways by an evaluation circuit. For example, the voltage at the resonant circuit is decoupled galvanically or inductively and rectified. The mean value of the rectified voltage over the period between two successive DC pulses stimulating the resonant circuit is a measure of the decay period and thus of the respective approach. Another evaluation option mentioned in DE-OS 33 18 900 consists in checking at certain points in time between two successive DC pulses that excite the oscillating circuit whether the oscillation is still ongoing or whether it has already subsided.

According to the explanations in DE-OS 33 18 900, the current consumption of the proximity sensor is so low that operating times of more than one year can be achieved without changing the battery when feeding from small galvanic elements (button cells). For certain applications, however, operating times of more than 5 years without changing the battery are desirable: For example, water meters and heat meters require maintenance-free operation for 5 years. So far, this requirement has not been met with button cells as batteries.

The invention is therefore based on the object of further reducing the power consumption in the simplest possible manner in a proximity sensor of the type mentioned at the outset.

This object is achieved by a proximity sensor with the features specified in claim 1. Advantageous developments of the invention are the subject of the dependent claims.

By using a transformer instead of a single coil, a voltage amplification of the decaying oscillation is achieved, which depends on the transformation ratio of the transformer. The associated increase in the sensitivity of the proximity sensor, conversely, allows that the primary side in the

To reduce the resonant circuit energy that is fed in, which leads to a longer service life in battery operation. A further increase in sensitivity of the proximity sensor can be achieved by using a shell core for the transformer; this is a cup-shaped core, in particular made of a ferrite material, with a pin arranged coaxially therein, which carries the two transformer windings. Such a shell core has a pronounced directional dependency in its sensitivity to vapor deposition from outside: the damping effect is particularly strong when the electrically conductive damping element is brought up to the open end of the shell core. The damping has its cause in eddy currents which are induced in the electrically conductive damping element when it approaches the transformer. On the other hand, the proximity sensor according to the invention is quite insensitive to magnetic interference: when a ferrite approaches the windings, the response signal of the sensor practically does not change. This insensitivity to magnetic interference is an advantage of the proximity sensor according to the invention.

That caused by the transformer voltage gain of the response signal of the proximity sensor caused because the only inductive coupling between the two ^{'Transformator¬} developments not covered significant electricity consumption, it will be abundantly overcompensated rather by the increase reached Empfindlichkeits¬.

The oscillation circuit can be excited by short electrical pulses occurring at equal time intervals, the time interval of the pulses being able to be adapted to the respective intended use of the proximity sensor, taking into account that the time interval of the oscillation circuit stimulating impulses should be at least so large that the excited damped vibrations have at least decayed in this period. An adaptation to different evaluation circuits, which may require different input amplitudes, can be done by adapting the transformation ratio of the transformer.

The electrical impulses that excite the resonant circuit do not have to occur periodically _? their occurrence can also be controlled externally, for example as a function of movement sequences in machines or devices in connection with which such a proximity sensor is used. In such applications in particular, it can be advantageous to generate the pulses for the excitation of the resonant circuit by means of a Wiegand pulse generator. Such a Wiegand pulse generator is disclosed for example in DE-OS 21 57 286. It has the advantage that it does not require an electrical power source to generate the electrical pulses, so that the power consumption of the sensor can be reduced further (only a power supply is then required for the evaluation circuit.)

The nature and particular sensitivity of the proximity sensor according to the invention mean that, despite the extremely low power consumption, numerous periods can still be observed in the decaying oscillation of the oscillating circuit. This enables a particularly elegant development of the invention

Type of evaluation of the decay behavior by means of a counter circuit which counts the number of periods in which the decaying oscillation in the secondary winding Has amplitudes above a predetermined threshold. This threshold is set in such a way that any interference pulses that occur do not contribute to the count result. The count result is a direct measure of the decay - duration of the damped oscillation. The advantage of such an evaluation is that it can be carried out with simple, inexpensive, fast, digital, electronic circuits which have only a minimal current consumption. The evaluation can be carried out, for example, by evaluating and registering a damping that is clear according to the circumstances of the application as a signal for the approach of the damping element, for example the occurrence of a maximum of 5 periods of the decaying oscillation above the predefinable threshold instead of 10 periods in the undamped Case.

However, it is even possible to derive a quantitative statement about the degree of proximity of the attenuator from the number of periods of the decaying vibration, so that the proximity sensor can be used not only as a threshold value detector but also as a displacement sensor. This applies in particular if a shell core is used as the transformer core, which gives the sensor a pronounced sensitivity in the direction of the longitudinal axis of the shell core. The particular suitability as a displacement measurement sensor is also directly related to the fact that by using a transformer instead of a single winding in the resonant circuit, the sensitivity of the sensor is increased and the number of periods of decaying vibration in the secondary winding of the transformer is increased. (The secondary winding behaves like an energy store; due to lower damping, the decaying oscillation leads to a greater number of periods).

Instead of determining the number of periods until the amplitude of the decaying oscillation drops to a predetermined threshold value, one could also conversely determine the voltage drop that the amplitude of the decaying oscillation suffers during a predetermined number of periods; such an evaluation

however, is more complex.

Proximity sensors according to the invention are also particularly suitable for use in a speed sensor in which an attenuator rotating with the speed to be determined approaches the core of the transformer as a result of its rotation and moves away from it. Each approach can be determined by the proximity sensor, reported and then registered. A special application of such a speed sensor equipped according to the invention can be found in water meters and heat meters and other flow meters, which like. these measure the flow of a flowing medium, for example by means of an impeller, and convert it into a rotary movement. Because of the extremely low current consumption of a proximity sensor according to the invention, water meters and the like quantity meters can be operated with a button cell over a period of 5 years and more without changing the batteries.

With a suitable design of the damping element, it is also possible to distinguish between forward and reverse running in such a speed sensor if the damping element is designed asymmetrically with respect to the reversal of the direction of rotation. You could, for example, form the attenuator triangular and lead such that it moves with one of its corners when moving in one direction of rotation approaches the sensitive side of the proximity sensor, while in the opposite direction of rotation it approaches one side of the sensitive side of the proximity sensor. In the first case, the damping starts slower than in the second case, so that in the first case more periods can be counted in the decaying oscillation than in the second case, from which the direction of rotation can be seen. Alternatively, the damping element could be designed in two parts and a first, weakly damping area and a second, strongly damping area could be provided and the two areas separated by an air gap. One then observes when the damping member rotates in one direction first a weakly damped and then a strongly damped oscillation, when rotating in the opposite direction first a strongly damped and then a weakly damped oscillation, from which the direction of rotation can also be seen.

An embodiment of the sensor according to the invention is shown schematically in the accompanying drawings,

FIG. 1 shows a longitudinal section through the sensor,

FIG. 2 shows a view of the opened sensor from the rear,

Figure 3 shows the arrangement of such a sensor in a water meter, and Figure 4 shows a block diagram for such a sensor.

In the sensor shown in FIGS. 1 and 2, in a cylindrical housing 1 at one end there is a transformer with a shell core 2, which has two windings L. and L ₂ on top of one another on its central pin 3, one of which is the secondary winding L. has a larger number of windings than the primary winding L,. The pot-type core 2 is arranged so that its open end in the same direction as the Ge ^'housing 1 of the sensor with its front end. The shell core 2 is fixed in the housing 1 and protected from the outside by a synthetic resin casting compound 4. In the middle part of the case

A circuit board 5 is arranged, which carries the excitation circuit for the oscillator. The excitation circuit are shown in FIG. 1 and in plan view

2 shows the following circuit elements: a quartz oscillator 6 with its two connection points 7 and 8; an integrated circuit 9, which forms a pulse signal with a suitable frequency and current strength from the signal of the quartz oscillator by frequency division and signal amplification; the connection points 10 and 11 for a DC voltage source; the connection points 12 and 13 for the primary winding L, and the connection points 14 and

II for the secondary winding L ₂ . At the connection point 14, the response signal of the proximity sensor is present in the form of a damped oscillation, which is an evaluation circuit is supplied, which can be accommodated on a second circuit board (not shown) in the housing 1, but can also be located outside the housing. In the latter case, a line 15 leads from the connection point 14 to the rear of the housing 1. Above the circuit board 5 there is also space for a button cell 16 for the current supply of the sensor. The button cell 16 is only indicated by dashed lines in FIG. 1.

FIG. 1 also shows the connecting lines 17 between the transformer and the printed circuit board 5.

The proximity sensor is provided on its front section with an external thread 18, with the aid of which it can be screwed into a suitable bore of a device, for example into the housing of a water meter, as shown in FIG. 3, on a collar surface 19.

The water meter shown in FIG. 3 has a housing 20 with an inlet connection 21 and an outlet connection 22. An impeller 23, which is driven by the water flow, is freely rotatably mounted in a measuring chamber lying between these two connections.

In the view shown in FIG. 3, the impeller is located under a partition wall 24 and is therefore only shown in broken lines. In the room Above the housing partition 24, a damping member 25 is freely rotatable about an axis 26 which extends coaxially to the axis of the impeller 23. The damping member 25 rotates synchronously with the impeller 23 and is coupled to it, for example, by means of a contactless magnetic clutch.

A proximity sensor 27 according to the invention is also installed in the housing partition 24, in such a way that its longitudinal axis, which coincides with the winding axis, runs parallel to the axis of rotation 26 of the damping element 25. The proximity sensor 27 is arranged in such a way that the damping member 25 is moved once over the front of the proximity sensor during each revolution and damps it in the process. Each passage of the attenuator 25 is recognized on the basis of the damping of the decaying vibration in the proximity sensor and can be registered. In order to be able to recognize whether the attenuator runs to the right or to the left, it is designed as a rotating triangular plate which, in the direction of rotation to the right with a tip ahead, approaches the proximity sensor 27, while when it is turned to the left with a radially running side of the triangle the proximity sensor 27 is approximated. In the first case, the

Damping of the sensor slower than in the second case, so that in the second case the damped oscillation decays faster than in the first case, whereby the direction of rotation can be recognized and the respective rotation can be counted positively or negatively accordingly. FIG. 4 shows a block diagram for a proximity sensor as shown in FIGS. 1 and 2 and can be used in a water meter according to FIG. The sensor is operated from a direct current source, in particular from a battery with the battery voltage U ". The excitation circuit for the primary winding L _{1 of} the transformer comprises a quartz oscillator 30 (shown in FIGS. 1 and 2 and designated by the reference number 6), the output signal of which is fed to a divider circuit 31, which results in a pulse train with a lower pulse frequency forms, which is matched to the respective application of the sensor.

The DC voltage pulses coming from the frequency divider circuit 31 are fed after amplification in an amplifier circuit 32 to an LC series resonant circuit consisting of a capacitor C and the winding L ^, which is excited by each of the pulses to a damped oscillation. The damped vibration is transmitted inductively into the secondary winding L _{2 of} the transformer, the primary winding of which is L. The decay behavior of the damped oscillation occurring in the secondary winding L ₂ is observed by an evaluation circuit, which consists of a discriminator circuit 33 and a counter circuit 34. The discriminator circuit 33 gives the U

damped oscillation in the winding L ₂ , which exceeds a predetermined threshold value, an electrical standard pulse to a counter circuit 34, which counts the pulses occurring during each damped oscillation and compares their number with the number of pulses occurring with the undamped sensor and from this comparison concludes the existing or nonexistent damping. The damping events determined by the counter circuit 34 can be fed in a manner known per se to a display or registration device 35, for example a roller counter or a liquid crystal display.

In an example of the sensor for use in a water meter, a quartz oscillator 30 with a frequency of 32 kHz with low power consumption was selected; a pulse train with a frequency of 256 Hz was formed from the output signal of the quartz crystal 30 by the divider circuit 31. After amplification by the amplifier circuit 32, these pulses had a height of 1 volt at a current of almost 1 μA. A capacitance of 400 pF was chosen for the capacitor C. The primary winding L. was carried out with 140 turns, the secondary winding L ₂ with 800 turns.

Claims

Claims:

1. Inductive proximity sensor with an LC oscillation circuit excited by electrical impulses, which can be treated by approaching an electrically conductive object (attenuator), and with an evaluation circuit for determining the oscillation damping caused by the attenuator, characterized in that the Inductance of the LC resonant circuit is formed by the primary winding L. of a transformer, the secondary winding L _{2 of which has} a higher number of turns than the primary winding L, and that the evaluation circuit (33/34) lies in the circuit of the secondary winding L ₂ .

2. Inductive proximity sensor according to claim 1, characterized in that the transformer core is a shell core (2).

3. Inductive proximity sensor according to claim 1, characterized in that a Wiegand pulse generator is provided for generating the pulses for the LC resonant circuit.

4. Inductive proximity sensor according to one of the preceding claims, characterized in that the evaluation circuit (33, 34) is a counting circuit which counts the number of periods in which the decaying oscillation in the secondary winding L _{2 has} amplitudes between predetermined values Has.

5. Use of a proximity sensor with the features according to one of the preceding claims in a speed sensor, in which an attenuator (25) rotating with the speed to be determined approaches the core (2) of the transformer as a result of its rotation and moves away from it away.

6. Speed sensor according to claim 5 and claim 2, characterized in that the axis of rotation (26) of the damping member (25) and the axis of the shell core (2) (which coincides with the winding axis) are parallel to each other.

7. Speed sensor according to claim 6, characterized in that the damping member (25) is designed asymmetrically with respect to its reversal of the direction of rotation.